Cholesteric liquid crystal displays are characterized by the fact that the pictures sty on the display even if the driving voltage is disconnected. The bistability and multistability also ensure a completely flicker-free static display and have the possibility of infinite multiplexing to create giant displays and/or ultra-high resolution displays. In cholesteric liquid crystals, the molecules are oriented in helices with a periodicity characteristic of material. In the planar state, the axis of this helix is perpendicular to the display plane. Light with a wavelength matching the pitch of the helix is reflected and the display appears bright. If an AC-voltage is applied, the structure of the liquid crystals changes from planar to focal conic texture. The focal conic state is predominately characterized by its highly diffused light scattering appearance caused by a distribution of small, birefringence domains, at the boundary between those domains the refractive index is abruptly changed. This texture has no single optic axis. The focal conic texture is typically milky-white (i.e., white light scattering). Both planar texture and focal conic texture can coexist in the same panel or entity. This is a very important property for display applications, whereby the gray scale can be realized.
Current cholesterics displays are utilizing “Bragg reflection”, one of the intrinsic properties of cholesterics. In Bragg reflection, only a portion of the incident light with the same handedness of circular polarization and also within the specific wave band can reflect back to the viewer, which generates a monochrome display. The remaining spectrum of the incoming light, however, including the 50% opposite handedness circular polarized and out-off Bragg reflection wave band, will pass through the display and be absorbed by the black coating material on the back surface of the display to ensure the contrast ratio. The overall light utilization efficiency is rather low and it is not qualified in some applications, such as a billboard at normal ambient lighting condition. The Bragg type reflection gives an impression that monochrome display is one of the distinctive properties of the ChLCD.
In many applications, human eyes are friendlier with full color spectrum, i.e., white color information written on the dark background. With the development of the flat panel display, more and more displays with neutral color have come into being, such as black-and-white STN display and AMTFT display, etc. Unfortunately, both of these approaches involve major disadvantages and limitations. The AMTFT displays are not true zero field image storage systems because they require constant power input for image refreshing. The STN displays do not possess inherent gray scale capability as a result of the extreme steepness of the electro-optical response curve of the display. To realize a gray scale, the resolution has to be reduced by using, for example, four pixels instead of one per area. Anywhere from one to four pixels are activated at a particular time to provide the gray scale effect. The AMTFT devices use semiconductors to provide memory effects and involve use of expensive, ultra high resistance liquid crystal materials to minimize RC losses. Additionally, these displays are both difficult and costly to produce and they are, at present, limited to relatively small size displays. The cholesteric display has many advantages over the STN and AMTFT display with its zero field memory effect, hemispheric viewing angle, gray scale capability and other optical performances, but it obviously needs to come up with black-and-white solution in order to keep its superiority.
U.S. Pat. No. 5,796,454 introduces a black-and-white back-lit ChLC display. It includes controllable ChLC structure, the first circular-polarizer laminating to the first substrate of the cell which has the same circular polarity as the liquid crystals, the second circular polarizer laminating to the second substrate of the cell which has a circular polarity opposite to the liquid crystals, and a light source. The black-and-white back-lit display is preferably illuminated by a light source that produces natural “white” light. Thus, when the display is illuminated by incident light, the circular polarizer transmits the 50% component of the incident light that is right-circularly polarized. When the ChLC is in an ON state, the light reflected by the ChLC is that portion of the incident light having wavelengths within the intrinsic spectral bandwidth, and the same handedness; The light that is transmitted through the ChLC is the complement of the intrinsic color of ChLC. The transmitted light has right-circular polarization, however, it is thus blocked by left-circular polarizer. Therefore, the observer will perceive that region of the display to be substantially black. When the display is in a OFF sate, the light transmitted through the polarizer is optically scattered by the ChLC. The portion of the incident light that is forward-scattered is emitted from the controllable ChLC structure as depolarized light. The left-circularly polarized portion of the forward-scattered light is transmitted through the left-circular polarizer, thus, is perceived by an observer. The black-and-white display, in '454 patent is generated by back-lit component and the ambient light is nothing but noise.
U.S. Pat. No. 6,344,887 introduces a method of manufacturing a full spectrum reflective cholesteric display, herein is incorporated by '887 teaches a cholesteric display employing polarizers with the same polarity as liquid crystals. The display takes advantages of two reflections: Bragg reflection (the first reflection) and metal reflection (the second reflection). The display utilizes a circular polarizer and a metal reflector film positioned on the backside of the display to guide the second component of the incoming light back to the viewer. However, the shortcoming of the angular dependence of the Bragg reflection and the unbalance of the two reflections made the display appearing a tint of color in the optical ON state, for example, greenish white. The reasons for that are described as follows:
Firstly, all the wavelengths of the Bragg reflection from the planar state is additionally characterized by optical activity for wavelengths of incident light away from central wavelength λo. When the cell structure is illuminated with ambient light and λo is in the visible spectrum, it reflects the lift to give an iridescent color with a narrow bandwidth and with the central wavelength λo for the normal incident, and for the varied angle of incident beam and the angle of reflective beam.
Secondly, in many cholesteric materials, the central wavelength of the Bragg reflection, λo, is also very temperature sensitive. A color shift will happen if the temperature changes from 0° C. to 50° C. In order to reduce the color shift, more twisting material with opposite handedness have to dope to the liquid crystals, thus reduces the overall performances of the liquid crystal, for example, increasing the viscosity of the liquid crystal and decreasing the clear point or working temperature range of the liquid crystal display.
Thirdly, the absorptive polarizer has limited transmission and polarization efficiency that causes the second reflection having less intensity than that of the first one, this property makes the liquid crystal display an additional unbalance in the two reflections.